The present invention relates to a control device for a transmission having a main transmission and a sub transmission.
In a conventional transmission including a main transmission which is combined with a sub transmission which can selectively change speed ranges of a speed changing mechanism, for example, a four-wheel drive transmission having a computer fed with data on the running state of a vehicle such as vehicle speed and throttle opening, a hydraulic controller supplied with the output of this computer, a main transmission having a plurality of speed ranges and controlled by this hydraulic controller, and a sub transmission changeable between high-speed and low-speed ranges, the speed of the main transmission is controlled on the basis of the vehicle speed, the throttle opening and a definite shifting characteristic given by the controller, being independent of the speed range of the sub transmission, while this is being changed from the high-speed to the low-speed or from the low-speed to the high-speed.
As described above, in a conventional automatic transmission consisting of main and sub transmissions, the speed range of the main transmission is controlled on the basis of the vehicle speed, the throttle opening and a specific shifting characteristic, being independent of the speed range of the sub transmission. However, if the shifting characteristic of the main transmission is so designed as to shift up earlier in order to reduce fuel consumption when the sub transmission speed is in the higher speed range suitable for ordinary and high-speed running, the main transmission is shifted up too early to generate enough torque at the driving wheels when the driver shifts down the sub transmission at the time of need for large torque at the driving wheels when climbing up a steep incline. Conversely, if the up-shift point of the shifting charaterictic of the main transmission is set at the higher side in order to generate large torque at the driving wheels when the sub transmission speed is in the low-speed range, fuel consumption is increased because of retardation of shifting up when the sub transmission speed is changed to the high-speed range when it is desired to make the vehicle run at ordinary and high speeds. Thus, there has been a problem in the conventional method.